Many vehicle tires, particularly automobile tires, carry an annular white stripe on their sidewall for aesthetic purposes. These white stripes are usually put on the tire as part of a separate strip that is extruded independently of the tread and sidewall components, and then laid over the appropriate sidewall when the assembled tire band is on the building drum. The white sidewall strip then becomes bonded to the main, black portion of the sidewall when the tire is cured in the mold. In this process, great care must be taken to apply the white sidewall strip uniformly around the tire and at the same time, press or "stitch" the strip firmly in place so that a good bonding occurs in the mold. Otherwise, the white sidewall on the finished tire might have a wavy appearance or eventually separate from the tire. The separate extruding operation and the care required in applying the white sidewall strip adds significantly to the cost of making the tire.
In spite of these difficulties, the option of co-extruding the white sidewall stripe integrally with the main body of the sidewall has generally not been considered feasible, because the sidewalls of automobile tires are usually co-extruded with the tread portions in a head that is already very complicated. The rubber used for the tread portion has a formulation that is different from the black rubber of the sidewall portion, and these portions usually flow through separate, intricate, carefully machined passages in the head, and they do not come together until reaching a final "preform" unit that has at its exit end a final die plate. Thus, it would appear to be very difficult and expensive to modify such a mixing apparatus so that a third rubber portion of a different color could be introduced at some point to form a uniform stripe in the middle of one of the sidewalls.
There have been die heads for forming three component strips used in the manufacture of white sidewall tires, for example the one shown in U.S. Pat. No. 2,569,373 to Fay. However, such extrusion apparatus was developed in the days when the white sidewall portion of the tire consisted of nearly the entire sidewall on one side of the tire. Thus, the white sidewall was not a stripe surrounded on either side by a black sidewall; the white portion of the sidewall was essentially tacked onto one side of the tread-sidewall unit. This enabled the white sidewall portion to be extruded through a passageway in the head that was in side-by-side relationship to the black sidewall passageway, with the single interface between the white and black sidewall portions being controlled by a tapered block between the two passageways (block 31 of U.S. Pat. No. 2,569,373).
U.S. Pat. No. 3,280,427 to Smith shows a two-component extrusion apparatus for making the kind of white sidewall strip that is formed separately and is applied later on the building drum to the top of the black sidewall. The center of such a strip is made of white rubber, with a thin layer of black rubber overlaying the white. The thin black layer connects the two black portions on the lateral sides of the white rubber. After the tire is formed and cured, the central portion of the thin black layer is buffed off to expose the white, and the width of the white sidewall strip on the finished tire is controlled by the width of the buffing wheel. The extrusion head shown in U.S. Pat. No. 3,280,427 for making this white sidewall strip is fairly typical of two component extruders and in fact is similar to, but on a smaller scale than, the two-component extruders that are used to make the tread and sidewall strips for tires.
U.S. Pat. Nos. 2,138,378 to Johnson, 3,715,420 to Kiyono, and 4,187,270 to Bartrum show apparatus extruding rubber or plastic products with one or more stripes. In these patents, the stripe material is forced into the main body of the extrudate, and the width of the strip is dependent on the varying pressure differential between the stream of strip material and the stream of main body material, which can result in an uneven stripe width. Likewise, the roll-applied beads applied to the wrapping material of U.S. Pat. No. 3,034,941 to Hessenthaler et al are subject to variations in pressures created by variations in wrapping material thickness and surges in the extruder that extrudes the bead material.
U.S. Pat. No. 3,959,432 to Wiley discloses an apparatus for forming a plastic laminated sheet in which a thin top layer of the laminate is maintained at a controlled thickness by a separator plate (numbered 34 in FIGS. 2, 3 and 4). However, this patent shows only the formation of a two-component sheet that has a base layer coated over its entire surface by a veneer material.